Liquid-crystal display devices are widely used as display devices for various information-processing devices including computers and television sets. In particular, TFT liquid-crystal display devices (hereinafter also referred to as TFT-LCDs) have become widespread and the market is expected to grow further. Consequently, there is demand for further improvements in image quality. Although a TFT-LCD will be taken as an example in the following description, the present invention is not limited to TFT-LCDs and is applicable to liquid-crystal display devices in general, including, for example, passive matrix liquid-crystal display devices and plasma address liquid-crystal display devices.
To date, the most widely used system for TFT-LCDs is a so-called TN (twisted nematic) mode in which liquid crystal with a positive dielectric anisotropy is aligned horizontally between substrates placed facing each other. The TN liquid-crystal display devices are characterized in that an alignment direction of liquid crystal molecules adjacent to one of the substrates are twisted 90° with respect to an alignment direction of liquid crystal molecules adjacent to the other substrate. The TN liquid-crystal display technology is an industrially mature technology and inexpensive manufacturing techniques have been established, but it is difficult for the TN liquid-crystal display devices to achieve high contrast ratios.
On the other hand, so-called VA liquid-crystal display devices are known in which liquid crystal with a negative dielectric anisotropy is aligned vertically between substrates placed facing each other. In VA liquid-crystal display devices, when no voltage is applied, since liquid crystal molecules are aligned in a direction substantially vertical to a substrate surface, a liquid crystal cell exhibits almost no birefringence or rotatory polarization, and consequently light passes through the liquid crystal cell almost without changing a polarization state of the liquid crystal cell. Therefore, if a pair of polarizers (linear polarizers) are placed above and below the liquid crystal cell such that absorption axes of the polarizers will be orthogonal to each other (hereinafter also referred to as cross-Nicol polarizers), an almost completely black display can be realized when no voltage is applied. When a voltage equal to or higher than a threshold voltage is applied (hereinafter simply referred to as “when a voltage is applied”), the liquid crystal molecules tilt and become substantially parallel to the substrate, exhibiting high birefringence and realizing a white display. Thus, the VA liquid-crystal display devices can easily achieve very high contrast ratios.
With such a VA liquid-crystal display device, if a tilt direction of the liquid crystal molecules when a voltage is applied is unidirectional, asymmetry appears in viewing angle characteristics of the liquid-crystal display device. To deal with this, multi-domain VA mode is widely used in which the liquid crystal molecules in the pixel are divided in terms of the tilt direction into multiple directions, for example, by devising the structure of pixel electrodes or providing protrusions or other alignment control means in the pixel. Regions of liquid crystal molecules differing in tilting orientation are also referred to as domains, hence the name “multi-domain VA mode” is also referred to as “MVA mode”.
In the MVA mode, from the viewpoint of maximizing the transmittance in a white display state, the axial orientation of the polarizer is normally set to be at an angle of 45° from the tilting orientation of liquid crystal molecules when a voltage is applied. This is because if α(in rads) is the angle between the axes of the polarizers and slow axis of the birefringent medium, the transmittance is proportional to sin2 (2α) when a birefringent medium is sandwiched between cross-Nicol polarizers. In a typical MVA mode, liquid crystal molecules can be divided in terms of tilting orientation into four domains: 45°, 135°, 225°, and 315°. In the MVA mode with division into four domains, schlieren orientation or orientation in unintended directions is often observed near domain boundaries or alignment control means, resulting in loss of transmittance.
To solve this problem, VA liquid-crystal display devices that use circularly polarizing plates are being studied (see, for example, Patent Document 1). With such liquid-crystal display devices, when a birefringent medium is sandwiched between a left-handed circularly polarizing plate and a right-handed circularly polarizing plate which are orthogonal to each other, the transmittance does not depend on the angle between the axes of the polarizers and slow axis of the birefringent medium. Consequently, even when the tilting orientation of liquid crystal molecules is other than 45°, 135°, 225°, and 315°, desired transmittance can be secured as long as the tilt of the liquid crystal molecules can be controlled. Therefore, for example, the liquid crystal molecules may be tilted in all orientations with a circular protrusion provided in the center of the pixel or may be tilted in random orientations without controlling the tilting orientation. Herein, the VA mode that uses circularly polarizing plates is also referred to as a circularly polarized VA mode or circular polarization mode. On the other hand, the VA mode that uses linearly polarizing plates is also referred to as a linearly polarized VA mode or linear polarization mode. Also, as is well known, the circularly polarizing plate is typically made up of a combination of a linearly polarizing plate and quarter-wave plate.
Furthermore, a circularly polarizing plate is known to have an optical antireflection function as follows: since circularly polarized light has the property of changing chirality between right and left when the circularly polarized light is reflected off a mirror, or the like, light incident, for example, upon a left-handed circularly polarizing plate placed on a mirror is converted into left-handed circularly polarized light by being transmitted through the circularly polarizing plate, and light reflected off the mirror is converted into right-handed circularly polarized light, which, however, cannot be transmitted through the left-handed circularly polarizing plate. Being capable of preventing unnecessary reflection when a display device is viewed in bright environments such as outdoors, the optical antireflection function of the circularly polarizing plate is known to have the effect of improving a bright-room contrast ratio of display devices including VA liquid-crystal display devices. It is believed that the unnecessary reflection is caused mainly by metal wiring and the like for transparent electrodes and TFT elements in the display device. Unless the unnecessary reflection is prevented, even if a display device realizes an almost completely black display when viewed in dark-room environments, the quantity of light during black display becomes excessive when the display device is viewed in bright environments, resulting in a reduced contrast ratio.
As described above, the circularly polarized VA mode that uses circularly polarizing plates can provide the effects of improving transmittance and preventing unnecessary reflection, but liquid-crystal display devices of a conventional circularly polarized VA mode have low contrast ratios at oblique viewing angles, and have room for improvement in that sufficient viewing angle characteristics are not available. Under these circumstances, various techniques for improving viewing angle characteristics using a birefringent layer (phase difference film) have been proposed. For example, Patent Documents 1, 2, 3, and 4 disclose methods (A), (B), (C), and (D) below, respectively, and Non-patent Document 1 discloses method (E) below.    (A) Method using two quarter-wave plates that satisfy the relationship nx>ny>nz.    (B) Method using a combination of two quarter-wave plates that satisfy the relationship nx>ny>nz and one or two second-class birefringent layers that satisfy the relationship nx<ny≦nz.    (C) Method using a combination of two quarter-wave plates that satisfy the relationship nx>nz>ny and a third-class birefringent layer that satisfies the relationship nx=ny>nz.    (D) Method using one or two half-wave plates in addition to the combination used in method (C).    (E) Method using a combination of two uniaxial quarter-wave plates (so-called A-plates that satisfy the relationship nx>ny=nz), a third-class birefringent layer that satisfies the relationship nx=ny>nz, and a birefringent layer that satisfies the relationship nx>nz>ny.